WATCH: WiFi in Active TV Channels

WATCH: WiFi in Active TV Channels
Xu Zhang and Edward W. Knightly
Rice University, Houston, Texas, USA
{Xu.Zhang,knightly}@rice.edu
Abstract
Keywords
Today’s “white space” model of spectrum sharing applied
in the UHF TV band allows channels that are not being
used regionally by a TV broadcaster to be re-purposed for
unlicensed-style secondary access in 24 hour increments. Unfortunately, populated areas have few unused channels for
white space usage. Nonetheless, from the UHF TV viewer’s perspective, Nielsen data show severe under-utilization
of this spectrum, with vast regions that are in range of
TV transmitters having no active TV receivers on multiple channels even at peak TV viewing times. In this paper,
we present the design, implementation, and experimental evaluation of WATCH (WiFi in Active TV CHannels), the
first system to enable secondary WiFi transmission even in
the presence of kilowatt-scale TV transmitters, while simultaneously protecting TV receivers when they are active. To
protect active TV receivers, WATCH includes a smartphonebased TV remote or an Internet-connected TV to inform
the WATCH controller of TV receivers’ spatial-spectral requirements. To enable WiFi transmission in UHF bands,
we design WATCH-IC (Interference Cancellation) and CAT
(Constructive Addition Transmission) to (i) exploit the unique environment of asynchronous WiFi transmission in the
presence of a strong streaming interferer, and (ii) require no
coordination with legacy TV transmitters. With FCC permission to test our implementation, we show that WATCH
can provide at least 6 times the total achievable rate to 4
watt secondary devices compared to current TV white space
systems, while limiting the increase in TV channel switching
time to less than 5%.
Spectrum Re-use; TV White Space; Database Controller;
TV Receiver Feedback; Interference Cancellation; Transmit
Beamforming
Categories and Subject Descriptors
C.2.1 [COMPUTER-COMMUNICATION NETWORKS]: Network Architecture and Design—Wireless communication
General Terms
Design, Experimentation, Performance, Reliability
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Copyright http://dx.doi.org/10.1145/2746285.2746313.
1.
INTRODUCTION
The UHF band of 400 MHz to 700 MHz is often termed
the “beach front property” of spectrum due to its superior
range and penetration compared to higher frequency bands.
Globally, this band is typically licensed to TV broadcasters, which can be considered as primary transmitters (or
primary users, PU) because they have the highest priority
to access the spectrum as protected incumbents. When a
geographical region has no primary broadcaster on a particular channel, that channel is said to be “TV white space
(TVWS),” which is available for transmission by secondary
users (SU) under today’s regulatory frameworks, e.g., in the
U.S. [9] and U.K. [15]. Unfortunately, the large number of
over-the-air TV broadcasters in many populated areas yields
extremely limited white space availability [13]. Nonetheless,
in practice, the number of viewers watching TV via UHF is
dwarfed by those watching via satellite or cable. For example, in the U.S., only 7% to 10% of all TV households rely
on over-the-air UHF broadcast for TV programming [5, 20].
In this paper, we design, implement and evaluate WATCH,
the first system that enables secondary WiFi transmission in
active TV channels. WATCH exploits the property that few
households are receiving UHF-band TV programming in a
given channel, time, and location. Because TV transmitters cannot be rapidly power-cycled even if they temporarily
have no receivers (due to high transmit power associated
capacitance), nor can they direct their energy only towards
active TV receivers, WATCH comprises the following three
contributions.
First, we propose a new spectrum sharing model and obtain an FCC license for its testing.1 To date, TVWS models
calculate exclusion zones (areas where secondary transmissions are not allowed/transmit power is set to zero) based
on transmitting TV channels and their corresponding tower
locations [9]. In contrast, we propose a dynamically computed exclusion zone characterized as the union of locations
where secondary user transmit power must be reduced in
order to protect active TV receivers. By exploiting that the
receiver-based exclusion zone has a much smaller footprint
1
FCC experimental license call sign WH9XHJ and file number 0121-EX-ST-2014.
Exclusion zone
Exclusion zone
TV transmitter
Inactive TV
TV transmitter
Inactive TV
Active TV
SU-TX
Inactive TV
SU-TX
SU-RX
Active TV
SU-RX
Inactive TV
TV service area
TV service area
(a)
(b)
Figure 1: The exclusion zone is computed with (a) TV transmitters in TVWS systems and (b) active TV receivers in WATCH.
than the transmitter-based exclusion zone, WATCH enables
vastly increasing secondary spectrum re-use.
Second, to protect active TV receivers from secondary
transmissions, we introduce two mechanisms to dynamically control the exclusion zone: (i) By generalizing the functionality of the spectrum database controller in standards
such as IEEE 802.11af [10], we design the WATCH spectrum
database controller to collect information of active TV receivers and accordingly coordinate secondary transmissions.
Namely, with active TV receiver channel usage and location
information, the WATCH controller dynamically determines
the maximum transmit power for SU’s. (ii) We design a
WATCH TV receiver that can inform the controller of TV
viewing. We introduce two complimentary feedback mechanisms to allow use with legacy TV systems: first, we propose
a smart remote control coupled with a legacy TV, e.g., via
a smartphone. Upon switching the TV channel via infrared,
the enhanced remote also informs the WATCH controller of
the new selection. Second, we propose a smart TV coupled
with a legacy remote, in which the Internet-connected TV
informs the WATCH controller of the new selection.
Third, we design a novel secondary transmit-receive architecture that enables secondary WiFi transmission even
when the kilowatt-scale TV transmitters are broadcasting.
For secondary reception, we design an interference cancellation (IC) technique, WATCH-IC, which exploits the fact
that TV signals are always being broadcasted, unlike IC in
non-streaming-broadcast systems such as cellular or WiFi.
In particular, our design cancels TV signals without requiring their preambles to be known a priori such that WATCH
is compatible with any broadcast technology and is not specific to a regional TV coding scheme. For secondary transmission, we design CAT, a Constructive Addition Transmission scheme for secondary WiFi transmitters. CAT precodes
transmissions and computes beam weights of the secondary
transmitting antenna array to ensure that secondary signals
add constructively after WATCH-IC. It addresses the problem of inadvertent cancellation of secondary signals without
coordination with legacy TV systems. Moreover, we employ
selective feedback to reduce CAT’s overhead. Compared to
transmit beamforming in IEEE 802.11n, CAT adapts to continuous and strong interference.
Finally, we implement the key components of WATCH
and experimentally evaluate their performance with FCC
permission and have the following outcomes.
Protecting active TV receivers: Without WATCH, off-theshelf TV tuners incur an average delay of 1.86 seconds to
switch between UHF channels (the time between receiving
the command from the remote and displaying the new channel content on the screen). We show that WATCH’s TV re-
ceiver feedback process adds no more than 5% to the above
channel switching time.
Secondary WiFi transmission in active TV channels: We
build two-antenna secondary transceivers using WARP [25]
and its UHF-band radio boards [1] and implement both
WATCH-IC and CAT. We provide the first demonstration
of secondary transmission in active TV channels, including
under interference of the strongest DTV signals in our lab
location in Houston. With 16-QAM and no channel coding,
WATCH-IC alone enables an average BER of 2.4 × 10−3
for secondary transmission at 2 dB secondary signal SINR in a typical indoor environment. CAT further doubles
the percentage of zero-BER packets from fewer than 40% to
more than 90%. In the same setup, legacy IEEE 802.11af
techniques cannot decode any secondary packets. We also
show that larger sub-carrier density is needed in WATCH
for secondary transmission compared to current TVWS systems [10] in order to cancel long-distance TV interference.
Urban scale analysis: We provide an urban-scale datadriven analysis of WATCH with UHF spectrum usage data of a U.S. major city (Houston) [24], TV viewing data
from Nielsen [19–21], and WATCH parameters from our implementation. We find that with 1% active TV receivers
(among all TV households) per UHF channel, WATCH can
provide 6 times the total achievable rate to 4 watt secondary
devices compared to current TVWS systems. This represents 42% of the total achievable rate if all TV transmitters
were turned off.
The remainder of this paper is organized as follows. Sec. 2
compares legacy spectrum sharing with WATCH spectrum
sharing. Sec. 3 and Sec. 4 introduce how active TV receivers are protected and how secondary transmissions are
enabled, respectively. Our implementation and evaluations
are in Sec. 5. Sec. 6 discusses the related work. Finally,
Sec. 7 concludes the paper.
2.
WATCH ARCHITECTURE
2.1
Legacy Spectrum Sharing
Current TVWS regulations target exclusion of secondary
re-use based on locations where TV broadcast services are
available. A typical scenario is shown in Fig. 1a: the
secondary transmitter (SU-TX) and the secondary receiver (SU-RX) can only communicate outside the TV service
area. In order to calculate the exclusion zone, information
of all TV transmitters is stored in a database. SU’s are
required to query the database periodically for updated information of their operational parameters, e.g., per location
re-usable frequencies [10].
Different methods are employed to compute the exclusion
zone. According to the FCC, the exclusion zone is an area
where the TV signal strength exceeds a pre-defined value,
which is calculated by the TV service threshold, SU antenna
height, etc. Outside this area, fixed secondary devices are
allowed to transmit at up to 4 W EIRP (Effective Isotropic
Radiation Power/transmit power including antenna gains),
while personal/portable secondary devices are restricted to
100 mW EIRP, or 40 mW EIRP if there are TV transmitters
occupying at least one of the two adjacent UHF channels [9]. In comparison, Ofcom divides space into 100 m×100
m blocks with each one having a calculated maximum SU
EIRP [15].
2.2
WATCH Spectrum Sharing
Legacy spectrum sharing models (e.g., TVWS) protect a
region determined by TV transmitter. However, because
the percentage of active TV receivers relying on over-the-air
UHF broadcasts is very small [5,20], we can re-purpose spectrum even within the TV service area: (i) Spectrum in the
spatial gaps: In-between active TV receivers, we allow secondary transmissions without interfering with TV receivers.
(ii) Spectrum in the temporal gaps: When a TV receiver is
not tuned into a particular TV channel, we allow secondary
transmissions in that channel and in the region around the
TV receiver.
While current TVWS systems cannot re-use both of the
above spectrum opportunities, WATCH enables re-use by
dynamically deciding the per-channel exclusion zone based
on protection of only active TV receivers. As illustrated
in Fig. 1b, the TV transmitter location is now irrelevant
to WATCH, because only active TV receivers can trigger
secondary exclusion. The exclusion zone for each channel is
also dynamic and adapted each time a TV receiver is tuned
in or out of that channel.
TV
Signal
Primary
TV Receiver
Primary Feedback
PROTECTING ACTIVE TV RECEIVERS
In this section, we describe mechanisms to protect broadcast reception quality of active TV receivers. We present the
design of the WATCH spectrum database controller, and the
functions and realization of primary feedback.
3.1
Spectrum Database Controller
The WATCH database controller computes the operational parameters of SU’s based on active TV receivers coupled with implicitly determined exclusion zones. In particular, WATCH does not explicitly disallow secondary transmissions in certain areas. Instead, we divide the region into blocks and compute the maximum SU EIRP for each
block. Secondary transmission requests are disallowed only
in blocks where the maximum SU EIRP is zero. Current TVWS database already has the information of transmit
power and location of TV transmitters. We further require
that WATCH database also collects the location and channel reception of active TV receivers. Therefore, the maximum SU EIRP for each block can be computed with various
pathloss models.
Whenever a TV receiver i becomes active in channel c, the
SU
for
WATCH controller updates the maximum SU EIRP Sc,j
c
channel c and each block j that is within distance d from
TV receiver i. dc is only related to the channel and can
be computed as follows: In WATCH, we limit the maxiSU
mum SU EIRP to Smax
. We also obtain the minimum rePU
quired TV signal strength Sservice
min and TV signal SINR
∆T V SIN R within the TV service area from legacy standards, e.g., the ATSC DTV standard. Denote h(·) as the pathloss
of secondary signals, which does not need to be isotropical.
hmax (·) is the maximum pathloss over a certain distance. dc
is selected to satisfy
∆T V
Spectrum Database
Controller
Primary
TV Transmitter
3.
CAT
Secondary
Transmitter
Figure 2: WATCH system overview.
To realize the new spectrum sharing model, WATCH comprises the following components as shown in Fig. 2: a spectrum database controller adapted to receive primary receiver feedback and compute active-TV-receiver-based exclusion
zones, legacy TV receivers enhanced with the capability to
provide feedback to the database controller, and multipleantenna SU’s. For the PU part, because spectrum sensing
cannot detect activity of TV receivers, WATCH employs
primary feedback to collect information (current channel reception and location) when a TV receiver is tuned into a
UHF channel and accordingly triggers secondary exclusion.
For the SU part, WATCH multiple-antenna SU’s employ
WATCH-IC and CAT to enable secondary transmission under TV interference. These subsystems are described separately in the following sections.
+ ∆redundency =
PU
Sservice
min
,
SU
Smax · hmax (dc )
(1)
where an additional ∆redundency is added to represent the
aggregate interference from multiple SU’s. When updating
SU
Sc,j
, WATCH ensures that
SU
Sc,j
≤
SU
Signal
Secondary
Receiver
WATCH-IC
SIN R
(∆T V
PU
Sc,i
,
c
SIN R + ∆redundency ) · h(di,j )
(2)
PU
where Sc,i
denotes the mean TV signal strength at TV receiver i in channel c, which can be computed by the LongleyRice irregular terrain model (currently used by FCC [18]).
When TV receiver i is turned off or switched to another
SU
channel, all Sc,j
within dc distance are updated again by
the WATCH controller: either to a larger value restricted
SU
by another active TV receiver i0 or to Smax
.
All SU’s are required to provide their infomation to the
spectrum database controller in order to acquire the transmission parameters, exactly as in current TVWS systems.
3.2
Primary Receiver Feedback
WATCH employs primary feedback to connect active TV
receivers to the spectrum database controller and dynamically determine the exclusion zone. The main functions of
primary feedback are to inform the controller of TV channel
changes and to act as a fail-safe mechanism.
SU
In WATCH, the in-block maximum SU EIRP Sc,j
is dynamically set with different active TV receivers. For block j,
if all TV receivers within dc are switched to channels other
SU
SU
than c or turned off, Sc,j
is reset to Smax
(for either a TV
receiver i or a block j, calculations are limited to SU’s or
PU’s within distance dc ). However, channel changes of TV
receivers cannot be detected by external techniques such as
spectrum sensing. Therefore, we require active TV receivers to inform the database controller of the channel changes
through primary feedback.
After a TV receiver informs the controller that it is tuned
into a particular UHF channel c, the controller updates alSU
l Sc,j
within dc . However, if the active TV receiver is
nonetheless incurring excessive interference due to the errors
in either the collected data, (errors in locations of PU’s/SU’s,
errors in pathloss estimates, etc.), WATCH employs the following fail-safe mechanism: If a TV receiver infers that there
is excessive interference, the WATCH controller will gradually increase ∆redundency , grow the exclusion zone, and reSU
calculate Sc,j
, until that the TV receiver can successfully
decode the TV programming. If ∆redundency exceeds a predefined threshold ∆max
redundancy and the TV receiver still infers
being interfered, WATCH controller will consider that the
excessive interference is due to poor channel quality of TV
signals instead of SU interference.
3.3
Primary Feedback Subsystem
While the broadcasting TV signals and the secondary data
are sent in the UHF band, primary feedback can be transmitted out-of-band via WiFi, cellular, or wired connections
such as DSL, or in-band via a UHF feedback channel. We
propose two methods to implement primary feedback with
minimum modifications to legacy TV systems: (i) Smart
remote: Smartphones can control TVs via infrared, e.g.,
Samsung Galaxy S5 and HTC One M8.2 Consequently, smartphones can be used as combined feedback and remote
devices. (ii) Smart TV: Feedback can be sent via the TV’s
Internet access.
The required feedback in the previous discussion considers all channels (TV channels, UHF channels) as identical.
However, in practice, channels are divided into two types: a
physical channel which occupies 6 MHz bandwidth and a virtual channel which contains TV programming. Each physical channel can comprise several virtual channels. Therefore, when an active TV receiver is switched between virtual
channels but stays in the same physical channel, it does not
need to contact the controller. Feedback is required only
when the TV receiver is switched between physical channels. According to [8], TV viewers switch among virtual channels with an average of 2.3-2.7 times per hour. The rate
of physical channel switch cannot be larger, which indicates
that the primary feedback of channel switch will not be sent
very frequently. For the fail-safe mechanism, the value of
∆redundency determines how well active TV receivers can be
protected. A large initial ∆redundency reduces the amount of
feedback to trigger the fail-safe mechanism, whereas in the
meantime increases the possibility to excessively limit the
SU EIRP.
In order to analyze the quality degradation of broadcast
video reception that occurs immediately following a physical channel switch, we denote tlegacy as the time that a
legacy TV receiver takes to switch between physical channels, which includes physical signal decode, transport stream
demultiplexing and video data decode. WATCH increases
tlegacy to tW AT CH by adding an additional delay used by the
2
http://en.wikipedia.org/wiki/Infrared_blaster
PU-TX
SU-TX
HPU1
HPU2
HSU1
HSU2
SU-RX
Figure 3: WATCH employs two-antenna SU-RX to cancel
TV signals.
TV receiver to send the primary feedback and the WATCH
SU
controller to update Sc,j
and contact SU’s. A comparison
between tlegacy and tW AT CH will be given in Sec. 5.
One concern of this subsystem is that some TVs lack the
ability to locate themselves. However, since a TV receiver
usually has a fixed location, we can register that location
once and use it for subsequent primary feedback. Registration of TV location is already required in some countries,
e.g., in Norway [8]. Moreover, even if a TV receiver cannot use the above two methods to provide real-time feedback, we can use its previously stored location to determine
a quasi-static protection zone and only update it occasionally. Another concern is the privacy of TV receivers. One
solution is to use data with larger granularity at the controller, e.g. grouping nearby TV receivers and using larger
time slot when updating activeness.
4.
SECONDARY NETWORKS IN RANGE
OF TV BROADCASTS
After a SU is given permission by the WATCH controller
to transmit, it can freely access the channel. However, new
mechanisms are needed to enable communication in the presence of TV transmitters, since TV transmitters cannot be
rapidly power cycled or adapt their energy footprint. Moreover, the maximum EIRP of a 30 m-high secondary device is
4 W over 1000 kW for a 200 m-high TV transmitter. In this
section, we show how WATCH exploits the unique properties of continuous TV interference to cancel TV signals at
the SU-RX and precode secondary transmission at SU-TX.
4.1
WATCH-IC: Cancellation of TV Signals
Receive beamforming uses multiple antennas to project
received signals onto the direction that is orthogonal to the
interference. When the interference is relatively strong, receive beamforming can lead to large SINR increase after
canceling most of the interfering signals.
A typical scenario of receive beamforming with a twoantenna SU-RX is shown in Fig. 3. WATCH does not require more than two antennas since there will only be signals from a single TV transmitter per UHF channel requiring
cancellation. Denote XP U and XSU to be the primary and
secondary signals, respectively. The two signals Y1 and Y2
at the two receiving antennas are
Y1 = HP U 1 XP U + HSU 1 XSU ,
(3)
Y2 = HP U 2 XP U + HSU 2 XSU .
If the SU-RX could receive a clean (uninterfered) preamble
of TV signals, it could estimate the primary channel state
information (CSI) HP U 1 and HP U 2 , and therefore cancel
XP U . However, this method cannot be applied to WATCH
due to the vast system-level heterogeneity between the primary and the secondary system (while the analysis hereafter
is focused on DTV signals, WATCH techniques can also be
applied to analog TV signals): (i) The ATSC DTV stan-
PU-TX
HPU2
HSU11
HSU21
α HSU12
SU-TX
WATCH-IC/CAT
HPU1
SU-TX
S
DATA
PU-TX
SU-RX
HSU22
WATCH-IC
β
SU-RX
F
A
Figure 5: CAT’s timeline.
Figure 4: WATCH employs CAT to improve the performance of WATCH-IC.
dard [2] uses single-carrier transmission. Its preambles (field
synchronized signals) are only defined for in-phase components. In contrast, the secondary system uses multi-carrier
OFDM transmission, for which preambles are defined for
in-phase/quadrature components. Moreover, for some subcarriers, there may be no preambles for estimating the CSI
of TV signals. (ii) Secondary signals may use different bandwidth from the 6 MHz DTV signals, e.g., a wider bandwidth
through channel bonding or occupying only part of the 6
MHz channel. (iii) Preambles of DTV signals are sent only
every 24.2 ms, which is inconvenient for the SU to use.
As a result, WATCH cancels TV signals without estimatHP U 1
, which
ing HP U 1 and HP U 2 . Instead, we estimate H
P U2
does not need to use preambles of TV signals and therefore
does not require synchronization between the primary and
the secondary system. That is, the SU network operates
fully asynchronously. In particular, we exploit that the TV
transmitter is always transmitting whereas SU-TX transmits intermittently. Consequently, when SU-TX is not sending
data and XSU = 0, SU-RX can estimate
HP U 1
Y1
=
HP0 U/SU −RX =
.
(4)
Y2
HP U 2
When SU-TX is sending data, TV signals can be canceled at
SU-RX by computing Y = Y1 − HP0 U/SU −RX Y2 . To realize
the receiver signal processing, we can use ZF (zero-forcing)
IC by Eq. (4) [11]. We can also use MMSE (minimum mean
square error) IC by computing HP0 U/SU −RX = CY1 Y2 CY−1
2 Y2
when XSU = 0, where C is the covariance matrix.
To generalize, M antennas at SU-RX can cancel N1 TV
signals and support N2 secondary data streams as long as
N1 + N2 ≤ M . In practice, N1 usually equals 1 due to the
deployment of broadcasting TV systems.
4.2
CAT: Constructive Addition Transmission
of Secondary Signals
Transmit beamforming is a method that adapts transmitter antennas’ gains and phases to focus signal energy onto the receiver. Unfortunately, this technique alone would
provide little benefit to WATCH due to the strong interference from TV transmitters. Consequently, we design CAT,
a Constructive Addition Transmission scheme that maximizes secondary signal SINR at SU-RX after accounting for
the channels from the TV transmitter to SU-RX.
Namely, without CAT, when WATCH cancels TV signals,
secondary signals may be inadvertently canceled as well in
some sub-carriers. This effect can be severer when the subcarrier density for secondary transmission becomes large and
when TV signals cannot be completely canceled. Indeed,
our experiments in Sec. 5 show that most bit errors of secondary transmissions are focused in several sub-carriers. To
address this problem, CAT leverages multiple antennas at
SU-TX to rotate the secondary signals at SU-RX to max0
imize the strength of its effective channel, which is |HSU
|
and it includes the WATCH-IC process.
As shown in Fig. 4, α = cos θ and β = ejφ sin θ are the
two beam weights. Different from Eq. (3), the two receiving
signals Y1 and Y2 now become
Y1 = HP U 1 XP U + (αHSU 11 + βHSU 12 )XSU ,
(5)
Y2 = HP U 2 XP U + (αHSU 21 + βHSU 22 )XSU .
0
After WATCH-IC, HSU
can be calculated as
0
HSU = (αHSU 11 + βHSU 12 ) −
HP0 U/SU −RX (αHSU 21 + βHSU 22 ) .
(6)
If all the CSI in Eq. (6) are known, we can calculate the
0
optimal α
ˆ and βˆ to maximize |HSU
|. However, to estimate
the CSI, SU-RX needs to receive clean preambles of secondary signals, which is impossible in WATCH due to the
continuous and strong TV signals.
To solve this problem, we define
H1 = HSU 11 − HP0 U/SU −RX HSU 21 ,
(7)
H2 = HSU 12 − HP0 U/SU −RX HSU 22 .
0
= αH1 + βH2 . Since there are only two
Observe that HSU
unknowns H1 and H2 , we can obtain their values by using
two sets of α and β.
An illustrative timeline of CAT is shown in Fig. 5:
(i) First, a sounding packet which contains training sequences with two different (α, β) sets is sent from SU-TX
to SU-RX.
(ii) At SU-RX, after TV signals are canceled, we can es0
and compute H1 and H2 . Then the optimal φˆ
timate HSU
and θˆ for α
ˆ and βˆ can be calculated as
φˆ = arg H1 − arg H2 ,
(8)
.
θˆ = π − arccos √ |H2 |
2
|H1 |2 +|H2 |2
(iii) The values of φˆ and θˆ are sent from SU-RX to SUTX. Similar to Eq. (4), at SU-TX HP0 U/SU −T X is estimated.
TV signals are canceled before the secondary feedback data
are decoded.
(iv) SU-TX uses φˆ and θˆ for precoding. At SU-RX,
WATCH-IC is used to cancel the TV signals.
4.3
Selective Sub-carrier Feedback
While CAT reduces BER, it also requires overhead. As
shown in Fig. 5, explicit sounding and feedback packets
need to be sent before CAT SU data transmission.
One way to reduce overhead in obtaining the CSI at the
transmitter is to use implicit sounding, which does not require feedback packets: Instead, SU-TX overhears packets
from SU-RX and employs channel reciprocity to estimate φˆ
ˆ Unfortunately, implicit sounding cannot be used in
and θ.
WATCH due to TV interference. Namely, TV signals are different at SU-RX and SU-TX. With different HP0 U/SU −RX
0
and HP0 U/SU −T X , SU-TX cannot estimate HSU
(at SU-RX),
which is required by CAT.
Therefore, WATCH employs an alternative to reduce the
overhead of collecting CSI. Generally, the feedback packets account for most of the additional overhead, because the
coded information of φˆ and θˆ of every sub-carrier need to be
sent to the SU-TX. Since secondary signals are not inadver-
TV
Signals
NULL
WARP SU-TX
Legacy TV
Transmitter
TV
Signals WARP SU-TX
SU
Data
Combine
Legacy TV
Transmitter
Off-the-shelf
TV Tuners
Primary
Feedback
(a) PU sub-system
WARP SU-RX
(b) SU sub-system
Figure 6: Implementation of WATCH.
tently canceled in every sub-carrier, CAT need only send selective φˆ and θˆ of those sub-carriers where secondary signals
are canceled, after which bit errors of secondary transmissions can still be largely reduced. Evaluations of CAT with
selective feedback are shown in Sec. 5.
5.
IMPLEMENTATION AND EVALUATION
To evaluate the performance of WATCH, we build a small scale indoor testbed and perform over-the-air experiments
with FCC permission. Moreover, with UHF spectrum usage and TV viewing data, we characterize WATCH’s performance on an urban scale.
5.1
Testbed Implementation
We implement the key components of WATCH and configure a testbed as follows: (i) The DTV systems are urban scale DTV broadcasters and the DTV receivers are
off-the-shelf TV tuners.3,4 (ii) We implement all aforementioned SU functionality on the software-defined radio platform WARP [25]. To enable UHF transmission and reception, we replace the default 2.4/5 GHz radio boards with
UHF-band radio boards designed by [1].
PU system. As shown in Fig. 6a, we combine over-theair DTV signals with secondary signals generated by WARP
and feed them into the TV tuners, which output TV programing to a laptop through the USB interface. To emulate the latency of primary feedback, we set a timer when
the channel switching command is sent to the TV tuners,
and stop WARP transmission by disabling the transmitting
chain after the timer expires.
SU system. As shown in Fig. 6b, we synchronize two
WARP boards to build a two-antenna secondary transceiver,
and construct a secondary link between secondary transceivers with 10 m separation. To download/upload signal samples
to/from WARP, we use the WARPLab 7 framework. At SUTX, we generate secondary packets according to the IEEE
802.11a standard, including preambles, pilots and data. The
SU-RX collects signals in a special format as shown in Fig.
7. The first part only contain TV signals with SU-TX not
transmitting. We employ them to calculate HP0 U/SU −RX for
WATCH-IC. The second part contain both TV signals and
header and payload of the secondary packet. We first cancel
TV signals with HP0 U/SU −RX . Then we correct the timing
and frequency offset of the secondary packet before decode.
Secondary signal SINR before WATCH-IC is calculated as
E{SP U SU } − E{SP U }
.
(9)
SIN R =
E{SP U }
3
4
DIAMOND ATI Theater HD 750 tuner
Hauppauge WinTV-HVR-950Q tuner
SPU
TV Signals
STS & LTS
SU Data
SPUSU
Figure 7: Collected data format at SU-RX.
5.2
Timing Requirement of Primary Feedback
Experiment Setup. To evaluate the interaction between the primary and the secondary system in WATCH,
we measure the channel switching time of off-the-shelf TV
tuners with different latencies of primary feedback. The
channel switching time is defined as the duration between
when the channel switching command is sent and when the
new TV programming is displayed. Our experiments address two issues: (i) How much latency can primary feedback
have so that WATCH’s increase in channel switching time is
overwhelmed by the inherent TV tuner’s channel switching
time? (ii) Can TV receivers begin the tuning process even
when the SU-TX is sending data?
To characterize the degradation of broadcast video reception quality, we define ∆ = (tW AT CH − tlegacy )/tlegacy , with
tlegacy and tW AT CH defined in Sec. 3. Three channels with
strong DTV signals are used in the experiments: 19 (503
MHz), 35 (599 MHz), and 42 (641 MHz). For the TV tuners, we find that there is a very narrow transition zone of
TV signal SINR between perfect TV programming displaying without errors and undecodable TV signals. We set the
WARP transmit power sufficiently high to create strong SU
interference, so that TV signals cannot be decoded if a SU
is transmitting.
Experiment Results. We vary the control-loop latency
of feedback between TV remote channel change indication
and notification to SU to vacate the corresponding channel. For each latency, we perform repeated experiments with
both TV tuners and different channel pairs and measure the
corresponding channel switching time for the TV tuner. The
results are depicted in Fig. 8: the x-axis shows the feedback
latency and the blue bars depict the measured average channel switching time, e.g., for zero feedback latency it is 1.86
s. The red line shows the sum of the zero-latency case result
and the feedback latency as depicted on the x-axis (i.e. 1.86
+ x) to provide a baseline for comparison. We only display
average channel switching time since results are close for
different TV tuners and channel pairs.
We make several observations: First, channel switch alone
is quite lengthy at 1.86 seconds even without WATCH. This
is because highly compressed TV programming results in
long initial decoding delay. Moreover, the TV tuner must
adapt to the new frequency and a potentially new TV signal
SINR. Second, the measured channel switching time with
WATCH (the blue bars) is smaller than the calculated sum
with feedback latency (the red line). This indicates that even
if the secondary signals are initially too strong to prevent the
decode of TV signals, the TV tuner can begin the tuning
process while the SU is still transmitting.
To estimate the latency of the primary feedback with a
smartphone-based remote, we consider an LTE scenario: According to [16, 26], the round-trip delay time (RTT) including both the access and core networks is approximately 35
ms. Such time can be divided into the uplink delay from the
smart remote to the database controller and the downlink
delay from the database controller to SU’s. Further measurements over LTE including end-to-end server response
3.0
2.5
2.0
1.5
1.0
Average BER
Channel switching time (s)
0
10
3.5
−1
10
−2
10
0.5
Without IC
With IC, 64 sub−carriers
With IC, 128 sub−carriers
With IC, 256 sub−carriers
With IC, 512 sub−carriers
−3
10
−14
0
0
100 200 300 400 500 600 700
Latency of the primary feedback process (ms)
−10
−6
−2
Secondary signal SINR (dB)
2
Figure 8: Measured channel switching time with different
primary feedback latencies.
Figure 9: Impact of sub-carrier density on MMSE WATCHIC.
delay to a large database server report about 80 ms average
RTT.5 Finding: In a full-scale system, the expected primary
feedback latency of WATCH will be less than 100 ms, which
leads to an additional channel switching time of TV’s with
∆ < 5%. Use of a wire-connected Smart TV can further
reduce ∆ due to smaller RTT.
However, after WATCH cancels the TV signals, the BER decreases, with larger sub-carrier density having a more rapid
decreasing rate (indicating better cancellation). At 2 dB
secondary signal SINR, the BER for 64 and 512 sub-carriers
is 1.9 × 10−2 and 2.4 × 10−3 respectively. In the experiments, while the increase of secondary signal strength after
WATCH-IC is similar, the cancellation degree of TV signals
vary significantly with different sub-carrier densities.
Generally, the required sub-carrier density is governed
by the delay spread (coherence bandwidth) of the signals, so that channel fading can be considered flat over an
OFDM sub-carrier. However, in WATCH, the required subcarrier density of secondary signals is dominated by the delay spread of TV signals. In our experiments, the distance
from the SU-TX to the SU-RX is only 10 m, whereas the distance from the TV transmitter to the SU-RX is 17 km. According to [17], the delay spread of the UHF band for indoor
WLANs is smaller than 1 µs, while that for tower-to-home
environments with tens of kilometers of distance is 11 to 25
µs. Therefore, even for short range secondary transmission,
in order to sufficiently cancel TV signals, a large sub-carrier
density is required. This sharply contrasts with the TVWS
standard: In IEEE 802.11af, SU’s only use 144 sub-carriers
for 6 MHz bandwidth (equivalent to 120 sub-carriers for 5
MHz bandwidth) [10]. Finding: To sufficiently cancel TV
signals with large delay spread, WATCH requires high subcarrier density even for short range secondary transmission.
We analyze the impact of other operational parameters of
the secondary system in [32] due to space limitations.
5.3
Cancellation of TV Signals
To evaluate WATCH under the most adversarial conditions, we sweep all UHF channels and select the one with
the strongest DTV signal, which is channel 26 (545 MHz) in
our lab location at Rice University. According to [24], the
TV transmitter of channel 26 is approximately 17 km away
from our lab and it can broadcast at a maximum of 1300
kW EIRP. In TVWS systems, this channel is clearly excluded from secondary transmission. Consequently, we received
an experimental license from the FCC to conduct the first
experiments of secondary transmission in active TV channels. Since channel 26 contains the strongest DTV signals,
our analysis shows the lower-bound performance of WATCH
in our lab. For evaluation, we separately evaluate WATCHIC and CAT, with this sub-section considering WATCH-IC
without CAT.
Experiment Setup. Because the primary (single-carrier)
and the secondary (multi-carrier) system use different modulation, it is important to determine the sub-carrier density (number of sub-carriers in certain bandwidth) for secondary transmission required by WATCH in diverse primary/secondary environments. SU transmissions in our experiments use 5 MHz bandwidth, 16-QAM and no channel
coding. For different secondary signal SINR, we vary the
transmit power at SU-TX. We also change the sub-carrier
density of SU transmission from 64 to 512. The sampling
rate and buffer size of WARP limit that we can use at most
512 sub-carriers for 5 MHz bandwidth.
Experiment Results. The results are shown in Fig. 9.
The x-axis is the maximum SINR of secondary signals at
the two receiving antennas before WATCH-IC (we do not
use average SINR since BER is more related to one of the
two receiving signals that has larger SINR). The y-axis is
the average BER of secondary signals. There are five curves
in the figure: The upper dashed curve shows the BER before
WATCH-IC, whereas the bottom four solid curves show the
BER after WATCH-IC.
Without WATCH-IC, the BER is near 0.5 (random guessing) indicating a complete failure if legacy systems are used.
5
http://www.fiercewireless.com/special-reports/3g4
g-wireless-network-latency-how-did-verizon-att-spr
int-and-t-mobile-compa-3
5.4
Constructive Addition Transmission of
Secondary Signals
In the following, we evaluate the performance of CAT coupled with WATCH-IC.
Experiment Setup. For repeatable experiments, we collect over-the-air channel traces and evaluate CAT with TV
signals received in channel 26. We use channel 29 (563 MHz)
to collect the CSI between SU-TX and SU-RX. According to
Google Spectrum Database, there are no co-channel TV signals in channel 29 in our lab, so that we can collect the
secondary CSI without TV interference. For trace postprocessing, 5 MHz secondary signals are generated with 512
sub-carriers, 16-QAM and no channel coding. The signals
are transmitted through secondary channels first and then
mixed with TV signals.
Out of the 512 sub-carriers, only 396 are used to transmit data/pilots (non-silent sub-carriers). To evaluate selective feedback, we only send φˆ and θˆ of N % of the 396
non-silent sub-carriers to the SU-TX. As shown in Sec. 4.2,
0.4
CAT, 100% feedback
CAT, 50% feedback
CAT, 1% feedback
Without CAT
0.2
0
0
2
5
BER
10
15
x 10
−3
1
0.8
0.6
0.4
CAT, 100% feedback
CAT, 50% feedback
CAT, 1% feedback
Without CAT
0.2
0
0
2
5
BER
10
15
−3
x 10
Cumulative Distribution Function
0.6
Cumulative Distribution Function
Cumulative Distribution Function
1
0.8
1
0.8
0.6
0.4
CAT, 100% feedback
CAT, 50% feedback
CAT, 1% feedback
Without CAT
0.2
0
0
2
5
BER
10
15
−3
x 10
(a)
(b)
(c)
Figure 10: Cumulative distribution functions of the BER of secondary packets at (a) 2 dB, (b) -2 dB, and (c) -6 dB secondary
signal SINR.
0
the SU-RX can estimate the current value of |HSU
|, which
0
is |HSU −current |. It can also calculate the optimal beam
weights for the SU-TX and thereby the maximum value of
0
0
|HSU
|, which is |HSU
−max |. Therefore, SU-RX, we can compute
0
|HSU
−max |
γ=
0
|HSU −current |
Table 1: Percentage of bit errors of the top M % data subcarriers with highest BER.
SINR (dB)
2
-2
-6
-10
-14
-18
M =1
50.0%
40.8%
19.2%
9.5%
5.5%
4.0%
M = 10
99.4%
98.9%
91.1%
58.2%
38.4%
28.8%
M = 50
100%
100%
100%
100%
99.6%
87.9%
(10)
for each non-silent sub-carrier. These γ’s are sorted and the
top N % sub-carriers with the largest γ are selected. Because
feedback accounts for most of the overhead, N % feedback
can be coarsely regarded as reducing the overhead to N %.
Note that both data and pilot sub-carriers are considered
for selective feedback. This is because pilots help to correct
phase offset. Therefore, inadvertently canceled pilots have
significant impact on signal decoding. We consider values
of N including 100 (all non-silent sub-carriers), 50 (the top
half), and 1 (top 1%).
Experiment Results. The cumulative distribution functions of secondary packet BER at different secondary signal
SINR are shown in Fig. 10. For Fig. 10a, 10b, and 10c,
the secondary signal SINR is 2 dB, -2 dB, and -6 dB, respectively. There is a vertical line of 2 × 10−3 BER in each
figure. In practical systems, forward error correction coding
can reduce 2×10−3 BER to below 2×10−16 [14]. Therefore,
we approximately view all the secondary packets with BER
smaller than 2 × 10−3 as largely error-free packets.
In Fig. 10, the red curves show the results when CAT is
not applied, while the black curves show the results when
CAT is applied with feedback of all sub-carriers. The results
indicate that CAT significantly increases the percentage of
error-free secondary packets. At 2 dB, -2 dB, and -6 dB
secondary signal SINR, it is increased from 79.0% to 96.2%,
37.7% to 92.9%, and 0.0% to 57.8%, respectively. In Fig.
10a, the percentage of packets with zero BER also increases from 35.2% to 90.8%. However, even with CAT, some
SU packets still have relative large BER. There are mainly two reasons: (i) When the residual TV signals are relatively strong compared to both |H1 | and |H2 | in Eq. (7),
CAT cannot provide significant gain. This is because even
if the secondary signals add constructively, the increase of
the secondary signal SINR is small. (ii) When channels vary
rapidly, the calculation of CAT may be stale.
There are two other curves showing the performance of
CAT with 50% (yellow curve) and 1% (blue curve) feedback. Note that with 1% feedback, we only send the beam
weights of 4 sub-carriers. At 2 dB secondary signal SINR, 50% feedback leads to 95.4% error-free packets, which is
very close to that of 100% feedback. Even with 1% feedback,
91.9% packets are error free. However, when the secondary
signal SINR decreases, the improvement by selective feedback also decreases. At -6 dB secondary signal SINR, the
percentage of error-free packets with 50% and 1% feedback
is only 22.6% and 2.0%, respectively. Both of them have
a large difference from the 57.8% error-free rate with 100%
feedback. The reason is shown in Table 1. M denotes the
percentage of data sub-carriers with highest BER (which is
different from N in Fig. 10 that considers data/pilot subcarriers). When the secondary signal SINR increases, bit
errors are more focused on the several sub-carriers. This is
because when the secondary signal SINR is large, most bit
errors are caused by inadvertent cancellation of secondary
signals in certain sub-carriers. However, when the secondary
signal SINR becomes smaller, more bit errors are caused by
residual TV signals.
Finding: (i) CAT significantly improves the performance
of secondary transmission even after WATCH-IC. (ii) For
cases of relatively high secondary signal SINR, CAT requires
feedback of only 1% non-silent sub-carriers. (iii) The improvement of CAT’s selective feedback is diminished when
the secondary signal SINR decreases, mainly because more
bit errors are caused by residual TV signals instead of inadvertent cancellation of secondary signals.
5.5
Urban Scale Analysis
Finally, we couple the in-lab measurements with UHF
spectrum usage and TV viewing data to estimate the performance of an urban scale deployment.
Setup. We collect TV signal strength of 20 strong UHF
channels in Houston (4th largest U.S. city) from TVFool [24].
Moreover, we use data of the total TV households in that city [21], the percentage of TV households that rely on broadcasts in the U.S. [20], and the percentage of TV households
that are watching a certain TV programming among all TV
households at peak TV viewing time in the U.S. [19].
In the data-driven simulation, we first divide the city into
blocks of 10 m×10 m. Active TV receivers are randomly
placed according to the calculated density. The maximum
EIRP of SU’s in each block is computed so that for all TV
receivers that can originally decode the TV programming
(TV signal strength larger than the TV service threshold),
the TV signal SINR is still above the TV SINR threshold.
If a PU and a SU are in the same block, we assume that
Normalized average
achievable rate in a channel
Table 2: Parameters for urban scale analysis.
SU
MAX SU EIRP [9] – Smax
4W
SU-TX, SU-RX/PU-RX Antenna Height
3/10 m
TV SINR Threshold
23+10 dB
– ∆T V
SIN R
[9]+∆redundency
PU
TV Service Threshold [9] – Sservice
min
-84 dBm/6 MHz
Noise Floor [9, 31]
-114 dBm/6 MHz
SU-SU/SU-PU Reference Distance
10/5 m
there is a SU-PU reference distance between them. We also
assume that SU-TX and SU-RX are separated by a SU-SU
reference distance. For the secondary signal pathloss, we
employ the Extended-Hata and Hata-SRD model [7]. Even
though here the coverage of each SU-TX is a sphere, in practice WATCH can be used with any non-isotropical pathloss
models. The achievable rates of secondary links in all the
blocks are calculated with Shannon equation, which are then
averaged over the whole urban area yielding spatial-spectral
efficiency results with unit bits/sec/Hz/m2 . Table 2 summarizes the parameters.
Results. Fig. 11 depicts the average spatial-spectral efficiency for all SU’s in one UHF channel. In particular, we
compute average bits/sec/Hz/m2 for each UHF channel and
present average values of the 20 channels normalized to the
achievable rate of a UHF channel that has no TV transmitters nor TV receivers. In other words, the normalization
baseline, 1, is the average achievable rate for SU’s in an
unused UHF channel by current TV white space regulations. There are two groups of bars in Fig. 11: the left one
shows the results when WATCH-IC and CAT are not applied, while the right one shows the results when WATCH-IC
and CAT are both applied. According to our experiments,
combined WATCH-IC and CAT can lead to approximately
20 dB increase of secondary signal SINR (for 512-sub-carrier
SU transmission). Within each group, the three bars from
left to right represent 0%, 1%, and 5% TV active (among
all TV households) in each UHF channel, respectively. The
case of 0% provides a baseline in which only the TV transmitter is on but no TV receivers are viewing the channel. In
this case, the secondary system is only constrained by TV
interference and not by the need to avoid interfering with
active TV receivers.
When WATCH-IC and CAT are not employed, the average achievable rate for SU’s is only 0.23 per UHF channel
in the case of 0% TV active rate. WATCH-IC and CAT can
almost double the achievable rate to 0.42 by increasing the
SINR of secondary signals. When there are TVs viewing the
channel, the spatial-spectral efficiency of the secondary system decreases due to the protection of active TV receivers.
When WATCH-IC and CAT are not applied (applied), compared to 0% TV active rate, 1% TV active rate results in
22.2% (12.7%) decrease of SU achievable rate, while 5% TV
active rate results in 46.6% (28.2%) decrease of SU achievable rate. However, while the number of active TV receivers
has a large influence on WATCH’s performance, in practice,
there cannot be an average of 5% TVs active in each UHF
channel since each household can view only a single channel
at a time. Therefore, the operational limit of WATCH is
primarily from strong TV signals (interference) rather than
protecting active TV receivers. Moreover, because TV signal strengths are different in different UHF channels, the
corresponding per channel achievable rate for SU’s are also
different.
0.5
0.4
0% TV active per UHF channel
1% TV active per UHF channel
5% TV active per UHF channel
0.3
0.2
0.1
0
Without WATCH−IC/CAT
With WATCH−IC/CAT
Figure 11: Normalized average achievable rates that
WATCH provides to SU’s in one UHF channel
Considering the case of 1% TV per-channel activity, the
corresponding average achievable rate for SU’s is 0.37 per
UHF channel. According to Google Spectrum Database,
Houston has only 3.26 out of the 47 UHF channels that
can be used by TVWS systems (spatially averaged), which
leads to a total normalized average achievable rate for SU’s
of 3.26. Compared to that, WATCH provides 0.37 × (47 −
3.26) + 3.26 = 19.44 total achievable rate of all the 47 UHF
channels. As an upper bound, if all TV transmitters were
turned off, the total achievable rate for SU’s would be 47
(1 per channel). Therefore, WATCH provides at least 6.0
times the total achievable rate to SU’s compared to current
TVWS systems, which is also over 42% of the maximum
value if all TV transmitters were turned off.
Finding: (i) WATCH can provide at least 6.0 times the
total achievable rate to SU’s compared to current TV white
space systems in Houston (typical of a large U.S. city). (ii)
The operational limit of WATCH is dominated by strong TV
signals, and not by the need to protect active TV receivers
as TV viewers have low viewing rates and only view one
channel at a time when active.
6.
RELATED WORK
Re-use of UHF channels. Most prior work on secondary re-use of UHF band employed the TVWS model,
in which TV transmitters determine the exclusion zone and
only “idle” channels can be re-purposed [3, 18, 30, 31]. Other work attempts to reduce the exclusion zone by excluding
indoor environments where thick walls can largely attenuate interference both from TV system to SU’s and from
secondary system to TV receivers [4, 28]. Ellingsaeter et
al. proposed to use TV receiver information to increase
SU spectrum re-use and estimated the resulting increase in
spectrum availability (in Hz) for several Norwegian cities [8].
In contrast, we realize the design, implementation, and experimental evaluation of WATCH to cancel TV signals at
SU-RX’s and protect dynamically active TV receivers from
secondary transmissions. Therefore, WATCH targets both
indoor and outdoor environments.
Interference cancellation. Previous IC work focused
on 2.4/5 GHz ISM bands [11, 23, 27]. Tan et al. decoded
overlapping WiFi packets [23]. Gollakota et al. proposed to
cancel interference without the help of preambles [11]. Based
on that, Yan et al. decode a WiFi packet and an overlapping
ZigBee packet [27]. In comparison, we design IC mechanisms
under the constraint of a streaming kilowatt-scale interferer
in the UHF band.
Constructive addition transmission. Transmit beamforming is employed in IEEE 802.11n and combined trans-
mit beamforming (interference alignment) and receive beamforming (interference cancellation) have been proposed for
WiFi bands [6, 12, 29]. However, such techniques require
coordination among different access points/clients. In contrast, TV transmitters are non-adaptive to the secondary
system in our scenario. CAT also operates under continuous and strong interfering TV signals.
Noam et al. proposed to send secondary signals in the
null-space of the interference channel of primary signals at
the primary receiver, so that interference to the primary receiver is minimized [22]. However, this technique requires
multiple-antenna PU and SU, with the primary transmitter
adaptively beamforming to the primary receiver according
to interference and channel conditions. In comparison, CAT
is compatible with legacy single-antenna broadcast TV systems. The purpose of CAT is also different, which is to avoid
inadvertent cancellation of secondary signals.
7.
CONCLUSION
In this paper, we propose WATCH, the first system to enable secondary WiFi transmission during active TV broadcasts. WATCH utilizes primary receiver feedback to protect
incumbent TV reception. We also design WATCH-IC and
CAT to enable secondary WiFi transmission under interference from streaming kilowatt-scale TV transmitters. We
build a testbed and evaluate WATCH with FCC permission
and show that in a typical U.S. major city, WATCH can
provide at least 6 times the total achievable rate to SU’s
compared to current TVWS regulatory models, while at the
same time only increasing TV channel switching time by less
than 5%.
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
8.
ACKNOWLEDGMENTS
The authors would like to thank Narendra Anand and
Ryan Guerra for their assistance in performing the experiments. This research was supported by Cisco Systems, Intel, the Keck Foundation, and by NSF grants CNS-1444056,
CNS-1126478 and CNS-1012831.
[22]
[23]
9.
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